John A. Jackman
Extension survey entomologist
The Texas A&M University System
Alsophila pometaria (Harris)
Also known as measuring worms, inch worms or loopers
Host : Has a wide host
range but prefers our Texas Live Oaks.
Cankerworms are a major
defoliator of broad leaf trees in east and central Texas. Many of the trees from
Corpus Christi to San Antonio, north to Dallas and throughout east Texas were
attacked in 1978 and 1979. Such outbreaks of cankerworms are not predictable and
reach high levels with little warning. Cankerworms pose a threat to broad leaf
trees. Early detection and control measures are necessary for proper control.
Two cankerworm species
are major problems in Texas: the spring cankerworm, Paleacrata
vernata (Peck); and the fall cankerworm, Alsophila pometaria
(Harris). The Spring cankerworms attack early in spring just as the leaves are
beginning to appear, or they sometimes attack the buds before the leaves open.
Other species also are present, but are similar in biology, appearance and
control. Cankerworms, in the family Geometridae, often are called inchworms or
measuring worms after their walking habit. They move by forming a loop with the
central part of the body and then extending the front to straighten out.
Adults are drab gray and green mottled moths. The males are fragile with a
wingspread of approximately 1 inch. Females are wingless, which means they can
migrate only by walking. The females lay eggs in the spring or fall, giving them
their common names. They crawl from the ground up tree trunks to lay eggs on the
lower side of the branches.
Larvae or caterpillars of
both species hatch from egg masses in the spring or fall when trees are just
reaching budbreak and new foliage is expanding.
Heavy defoliation usually occurs throughout April as long as the leaves
are palatable and tender and can cause growth loss, mast reduction and, if
coupled with other stresses, may result in mortality. Trees may be completely
stripped of foliage, some never having a chance to leaf out. The larvae grow to
approximately 1-inch long and are quite slender. Color is extremely variable,
with light and dark forms in both species. Both are striped longitudinally with
green, brown and pale yellow colors predominant. The spring cankerworm has two
pairs of prolegs on the abdomen and the fall cankerworm has three pairs; thus,
they can be distinguished readily (See Below ).
However, the drab colors make them blend with tree branches, which they resemble
when they sit still.
Larvae feed for about 3
or 4 weeks before they drop to the ground to pupate in the soil, usually in late
May or early June. Cankerworms remain in a pupal state in the soil until the
adults emerge in the spring or fall, depending on the species. They are a
nuisance when they drop to the ground because they leave silk threads trailing
from the trees. Dispersal to other
trees is accomplished, when the small caterpillars, which are attached to the
tree by a silken thread, which act as a kite and they are blown from tree to
After adults emerge,
females crawl up the tree trunks and onto the branches where they deposit eggs
in clusters of about 100 in bark crevices on the limbs or the trunk. The eggs
remain until the appropriate time to hatch in the spring. Fall cankerworm eggs
remain from November until March or April when they hatch.
Control is suggested for valuable
trees such as new transplants, specimen trees in a home landscape or fruit and
nut bearing trees. If a tree has low vigor or has had a previous history of
defoliation or other problems such as drought or disease, it is more likely to
be damaged by cankerworm defoliation and should be treated.
However, cankerworm control is not
always justified. Large, healthy trees can withstand total defoliation without
significant damage. Trees that are defoliated early in the year will leaf out
again and show no signs of the damage. Inspect trees in the spring. Both larvae
and damage should be present before control is justified.
Since female moths must crawl from the ground up into the tree, previous control
measures recommended trapping the insects by placing a band of, a
sticky substance known as “Tree Tanglefoot,” “Stickem” or Tack Trap” may be
spread around the trunk on a band of heavy paper 4-6 inches wide and 6-7 feet
high and relative to what the branches will allow for height and room. The bark
crevices beneath the paper should be filled with cotton. However, this method is
not reliable for total control since emergence and egg laying may be prolonged.
Also, tree bands would be needed in both fall and spring to control cankerworms
effectively, and small larvae can migrate easily from tree to tree by dropping
on a silken thread and blowing in the wind.
Moth laying eggs
Damage to leaves
The surest cankerworm control
measure is an insecticide timed to kill early larvae. Insecticides registered
for controlling cankerworms include: malathion; carbaryl ( Sevin® ); acephate (
Orthene® ); Imidan® ; and methoxychlor. In addition, the biological
insecticide, Bacillus thuringiensis Kurstaki Strain (
Dipel®, Crymax®: Bactur® & and Biotrol® ) also is effective for controlling
cankerworms. Considered the safest material, particularly around homes, this
biological insecticide or bio-pesticide may take several days to control
caterpillars since it is a disease organism. Small quantities of a detergent
added to Bacillus thuringiensis k acts as a spreader/sticker.
My preference is “B.t.” Bacillus
thuringiensis Kurstaki Strain is a highly
selective poison. It is ONLY effective against caterpillars,
moths and butterflies. Once eaten the protein breaks down into a compound that
paralyzes the caterpillar’s mouth and stomach, making it impossible for the
insect to eat. It starves to death in several days. In some caterpillars the
spores germinate into bacteria that cause an infection, leading to disease and
death in several days later. The components of crystals and spores that are
present in the “B.t.” formulation are not activated and do not become toxic
until eaten by the caterpillars. It is not toxic to humans , animals or birds.
Natural predators can consume the caterpillars without harm. We
also include a foliar
fertilizer with the spray application to help the tree to restore its vigor.
Spraying is not recommended once
caterpillars are near their full size. At that time both the caterpillar damage
and nuisance are nearly over and sprays are not beneficial.
label clearances are subject to change and changes may have occurred since this
publication was printed. The pesticide USER is always responsible for the
effects of pesticides on his own plants or household goods as well as problems
caused by drift from his property to other property or plants. Always read and
follow carefully the instructions on the container label.
Tree was partial defoliated by
cankerworms. It has spent a tremendous energy producing new buds and leaves.
Now under serious stress the tree will use up reserve energy to re-foliate
causing even greater stress and risk of mortality.
Tent Caterpillars |
Oak Leaf Roller
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understanding that no discrimination is intended and no endorsement by the
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